Bell-mouth flow tube is usually used to test the intake flow in aero engine tests. Aiming at the problem of the inaccurate measurement results and the increased measurement cost caused by the inaccurate layout of measuring points in the process of measurement, the velocity distribution on the measurement section of the inlet section of the tube is studied. The measurement principle of the bell-mouth flow tube and the flow state of the airflow in the tube under the turbulent flow state are analyzed in this paper. The variation of velocity along the diameter direction on the measurement section is studied, and the mathematical model of the radial velocity distribution in the tube is established. Finally, the velocity distribution model is analyzed and verified by the methods of experiment and simulation. A new method based on velocity distribution model is provided for accurately obtaining the air flow in the tube, that is, the data information of a few points on the measurement section of the flow tube is measured through the test to establish the velocity distribution model, so that a more accurate air flow value in the tube is obtained. The research results are of great significance to the measurement of intake air flow and the evaluation test of engine performance.